It might turn out that runtime checks are just not feasible, but in that
case I think we still need some way of solving the original problem:
that people are using sync calls and blocking up the main loop.

I'm all for discouraging sync use in the main thread after the
application is up, but
are stalled applications actually a wide spread problem? I don't
really remember any
apps I use regularly locking up (except for maybe hexchat when connecting to my
irc proxy). Granted, it's harder to notice these days now that we
have a compositor
and applications don't need to redraw after getting uncovered, so it
could be it's
happening more than is obvious. But, I just wonder if we really need
to do anything.
It seems like the bad/obvious cases would get bug reports and fixes
pretty quickly,
and so the problem should regulate itself.

There are quite often gvfs or Nautilus bug reports that say "network mount
causes desktop to slow down". I tracked it down to some gnome-shell
extension which somehow does sync calls to the remote fs which makes
everything crawl.
In general though I think severe warnings on the documentation for each sync
call is better than runtime warnings or compile-time warnings.
Ross